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Flat (Nonpolypoid) Colorectal Lesions Identified at CT Colonography in a U.S. Screening Population

Posted May 31, 2010 Updated Apr 7, 2023
By Perry J. Pickhardt MD , David H. Kim MD , Jessica B. Robbins MD
11 min read

Rationale and Objectives

The aim of this study was to investigate the clinical importance and height definition of flat (nonpolypoid) colorectal lesions detected on screening computed tomographic colonography (CTC).

Materials and Methods

Results from prospective screening CTC in 5107 consecutive asymptomatic adults (mean age, 56.9 years) at a single center were analyzed. All detected colorectal lesions ≥ 6 mm were prospectively categorized as polypoid or flat (nonpolypoid). The maximal height of all flat lesions was measured to assess the suggested 3-mm threshold definition.

Results

Of 954 polyps measuring ≥ 6 mm identified on screening CTC, 125 lesions (13.1%) in 106 adults were prospectively categorized as flat, with a mean size of 12.7 mm (range, 6–80 mm), including 73 lesions 6 to 9 mm, 42 lesions 10 to 29 mm, and 10 lesions ≥ 3 cm (carpet lesions). For polyps between 6 and 30 mm in size, flat lesions were less likely than polypoid lesions to be neoplastic (25.0% vs 60.3%, P < .001), histologically advanced (5.4% vs 12.1%, P = .07) or malignant (0% vs 0.5%, P = NS). Two of 10 carpet lesions (20%) were malignant, compared to 50% of polypoid masses ≥ 3 cm. Of nine flat lesions seen only on colonoscopy (false-negatives on CTC), two were neoplastic (tubular adenomas), and none was histologically advanced. For all flat lesions between 6 and 30 mm, the maximal height averaged 2.2 mm and was ≤3 mm in 86.1%, including 93.2% of small 6-mm to 9-mm flat lesions.

Conclusion

In a US screening population, flat colorectal lesions detected on CTC demonstrated less aggressive histologic features compared to polypoid lesions. Excluding carpet lesions, a maximal height of 3 mm appears to be a reasonable definition.

The true prevalence and potential clinical significance of “flat” or “nonpolypoid” colorectal lesions in the US screening population have been the source of recent debate . Some investigators have suggested that flat colorectal lesions may be more common and aggressive than previously thought. Unfortunately, a number of interrelated factors have combined to complicate the proper analysis of this issue, most notably the lack of standard definitions, but also potential geographic population differences (eg, East Asia vs the United States), mixing of high-risk and screening cohorts, varying detection strategies, and differences in histopathologic assessment. The term “flat” is actually somewhat of a misnomer, because truly flat lesions are extremely rare . Rather, this term generally describes lesions that are superficially elevated.

The issue of flat lesions is a potentially important consideration for computed tomographic (CT) colonography (CTC), particularly if this emerging test is to be used in population screening. As is the case with optical colonoscopy, flat lesions on CTC are generally less conspicuous than polypoid lesions of a similar size but may still detectable with appropriate technique and awareness.

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Materials and Methods

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Figure 1, Polypoid and nonpolypoid lesions identified within the cecum of the same patient on computed tomographic (CT) colonography. (a) Image from three-dimensional endoluminal CT colonography shows both a superficially elevated, plaquelike 21-mm flat lesion (arrowheads) and an adjacent 14-mm sessile polyp. Although both are detectable, the polypoid lesion is more conspicuous. (b) Three-dimensional endoluminal CT colonographic image centered on flat lesion with calipers placed for width measurement. (c,d) Magnified transverse two-dimensional images with polyp (c) and soft tissue (d) window settings show the height measurement of the flat lesion (2.8 mm; arrowheads). Although soft tissue windows are useful to confirm the soft tissue nature of a focal lesion, note how linear measurement is underestimated. All two-dimensional linear measurements are obtained with polyp windowing (width, 2000 Hounsfield units; level, 0 Hounsfield units). (e) Magnified two-dimensional image of sessile polyp shows that lesion height (6.6 mm) is actually less than half of its width, demonstrating that this definition is inappropriately inclusive. (f) Image from subsequent colonoscopy shows the flat lesion (arrowheads) prior to resection. Both polyps in this case proved to be tubulovillous adenomas without high-grade dysplasia.

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Results

Prevalence and Histologic Features of Polyps Detected on CTC

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Figure 2, Large superficially elevated hyperplastic polyp detected on screening computed tomographic (CT) colonography. (a) Three-dimensional endoluminal CT colonographic image shows a flat lesion (arrowheads) in the transverse colon, which measured 20 mm in width and 1.7 mm in maximal height. (b) Two-dimensional transverse CT colonographic image confirms the three-dimensional finding of a flat lesion (arrowheads). (c) Image of superficially elevated lesion on subsequent colonoscopy. The lesion proved to be hyperplastic, which is typical for subtle nonpolypoid lesions in our screening population.

Figure 3, Flat hyperplastic lesions resulting in false-negative interpretations on colonoscopy and computed tomographic (CT) colonography (CTC). (a–c) Three-dimensional endoluminal (a) and two-dimensional transverse (b) CT colonographic images show a flat 14-mm soft tissue lesion (arrowheads) situated along a colonic fold at the hepatic flexure. The lesion was not found on subsequent same-day colonoscopy, likely related to its right-sided location, but was again identified on follow-up CTC 15 months later (not shown). The lesion was confirmed at repeat same-day colonoscopy (c) and proved to be hyperplastic at pathologic evaluation. Such false-negatives on colonoscopy are often mistakenly presumed to represent false-positives on CTC. (d) Image from colonoscopy in another adult referred from screening CTC shows two subtle flat lesions (arrowheads) in the ascending colon that measured 10 and 20 mm, respectively. Only one of these lesions (presumably the larger one on the right) was identified prospectively on CTC. Both lesions proved to be hyperplastic.

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Morphologic Features of Flat (Nonpolypoid) Lesions

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Flat Lesions Identified Only on Colonoscopy (False-negatives on CTC)

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Discussion

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Academic Radiology, Volume 17, Issue 6
Journals General Radiology
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